This invention relates to fluid-gauging systems.
The invention is particularly concerned with capacitive gauging systems of the kind in which diodes or similar devices are connected with a capacitive sensor in a fluid tank to produce a unidirectional current output.
In capacitive fluid-gauging systems the level of fluid in a reservoir is determined by sensing changes in the value of a capacitor located within the container. The capacitor has two plates, separated by an air-gap which is filled or emptied as the depth of fluid changes, thereby altering its capacitance. An alternating electric signal is supplied to the capacitor, and its output, after rectification, is supplied to a suitable measuring circuit.
The diodes used to rectify the output of the capacitor are immersed in the fluid and are subjected to the temperature changes of the fluid. Typically, the temperature coefficient of such diodes is about -2 mV/.degree.C. Since the voltage drop across the diodes affects the output of the capacitor sensor as supplied to the measuring circuit, it is usually necessary to compensate for this. Compensation may be achieved by use of a temperature sensor mounted close to the diodes and with knowledge of the temperature coefficient of the diodes.
This method of compensation has the disadvantage of requiring the use of additional components leading to increased complexity and reduced reliability.